On the cover: Theories and beliefs about the cosmos have been nearly as vast as the universe itself, covering everything from the birth of a single star to the discovery of objects such as the Galaxy Triplet ARP 274 (pictured).
Early theories of the universe depicted Earth as a pillar at the center of everything, around which the Sun, Moon, and planets circled. The Greeks were experts at geography—they calculated the radius of Earth by using the Sun— and mathematics, but they used reason to avoid coming to some obvious conclusions. By the time of Aristotle (384–322 BCE), hundreds of years later, they believed Earth was in the shape of a ball, but still thought that the Sun, stars, and planets passed around Earth. Aristotle believed that there could only be a fi nite number of stars, because they all passed around Earth every 24 hours. ... Scientists now believe that about a quarter of the total energy density of the universe is composed of dark matter. ... The totality of the universe is perhaps unknowable. But the theories, beliefs, and laws examined in this book—which represent the life’s work of many incredible individuals—offer the best understanding humankind has been able to achieve thus far. It is awe-inspiring to ponder what advances the dreamers and visionaries of the 21st century will bring.